Planning the Project

Constructing the Sampling Plan

After the problem is defined, the planning process can begin. Analytical text books explain that you must consider the sample collection, sample storage, sample preparation, measurement, and reporting, along with any QA/QC requirements. With so many considerations, where should you start?

A synthetic organic chemist will construct a plan by working backwards from the final product. A similar approach may work well for the trace analyst. Start by examining the following basic information:

1. The analyte(s) of interest.
2. The required detection limit(s).
3. The uncertainty requirement(s).
4. The chemical composition (matrix) of the sample.
5. The quantity, availability, and history of the sample.

Much of the above list can be determined based on information gathered while defining the problem. In most cases, analytical resources are available in-house to address the problem. For example:

1. The basic information listed above is sufficient to determine whether publications or information is available in your reference library. Always start with a search of the literature.
2. The identity and detection limit requirement of each analyte indicates the analyte measurement technique(s) required and the amount of sample required.
3. The uncertainty requirement indicates the number of measurements, assuming there is sufficient sample available.
4. The chemical composition of the sample, together with the identity of the analyte(s), indicates possible sample preparation routes.
5. The identity of the analyte(s), together with the detection limit requirement(s), indicates the degree that contamination issues should be considered. This determines the need for analytical blanks and special apparatus or a clean area / room.
6. The sample composition indicates potential interference issues.
7. The sample composition or type indicates the uncertainty to be expected form the sample collection and/or the need to develop a sampling procedure and to determine sampling uncertainty. For example, the sample may be the only "world's supply", negating the need for a sampling procedure.
8. The estimated sampling uncertainty can be used to define the analytical measurement precision (i.e. -- reducing the analytical error to less than one third of the sampling error serves no purpose).

The basic information can provide the analyst with potential analytical measurement technique(s), suspected interferences, contamination issues, and the number of sample measurements required per determination (measurement refers to a complete analysis including sampling, preparation, instrumental analysis and reporting the final result and uncertainty). At this stage of the planning process, the analyst can determine if a certified reference material (CRM) should be obtained for method validation. In addition, the chemist can approximate the need for analytical reagents and apparatus and/or calibration standards.

Lastly, estimate the time and cost of the project and base your initial approach on these estimates. Remember, there is always the possibility that more than one iteration may be required before an acceptable approach can be developed.

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